In power transmission systems, such as in direct current (DC) power transmission systems, an input/output device or I/O unit is a device or unit which receives and transmits signals between field equipment, like circuit breakers and transformers, and control and protection computers. The signals can be analog signals, such as system measurement signals in which A/D conversion is performed. The signals can also be digital signals like control signals and status signals. For this reason the I/O units communicate with such control and protection computers via a field bus and with the field equipment via field wiring.
There exist some implementations where distributed I/O (input/output) units are used in HVDC (High Voltage Direct Current, FACTS (Flexible Alternating Current Transmission System) and AC substation applications.
Many such systems and applications are designed such that the connection points for field wiring are on the I/O units using circuit board mounted terminal blocks.
Such terminal blocks are not suitable to connect directly to the cables from the field. For example, they are difficult to adapt to the varying specific utility requirements and strict internal rules regulating the type of terminal block that are allowed in their stations.
There exist a number of documents describing connections of terminal blocks.
U.S. Pat. No. 7,553,199 describes a terminal block connected to a circuit board as well as to a DIN rail.
DE 203 19 657 describes an I/O unit with a box having a rail and openings for receiving field wires. The box is a terminal block arrangement with at least one input terminal block, a freely selectable number of connecting blocks and a bus terminating block, each with an insulating housing, several connectors and a free space for a circuit board with an electronic circuit.
DE 196 00 997 likewise describes terminal blocks on a DIN rail. A number of terminal blocks make up a bus subscriber, which includes a processing unit and senders and receivers for interconnecting several bus subscribers.
U.S. Pat. No. 6,575,771 describes a terminal block on a DIN rail, which can be connected to power supply and monitoring and control signal voltages via a bus connector unit and which can receive a circuit board. The terminal block also has terminal strips for connection to corresponding terminal strips on the circuit board.
U.S. Pat. No. 2009/0221166 describes an I/O wiring interface with a terminal block connector inside a housing. Inside this housing there is also an I/O module implemented through a circuit board.
U.S. Pat. No. 2007/0233323 also describes a modular I/O device that includes digital and analog input and output modules connectable to a each other and to a DIN rail.
However given the above mentioned problems, there is still a need for simplifying connection of terminal blocks with units providing the required I/O functionality.